A 28-year-old man with a known history of fasting-induced seizures since childhood is admitted with recurrent hypoglycemia (blood glucose 35–50 mg/dL) despite a 6-hour fast. His parents report that he requires frequent meals and cannot tolerate prolonged fasting. Liver ultrasound shows hepatomegaly with a smooth, non-cirrhotic appearance. Serum lactate is 8 mmol/L (normal < 2), and uric acid is markedly elevated at 9.2 mg/dL. Which of the following metabolic derangements is the PRIMARY consequence of the underlying enzyme defect?

Explanation

## Clinical Diagnosis: Von Gierke Disease (Type I GSD) This patient's presentation is **pathognomonic for glucose-6-phosphatase deficiency**: - **Severe fasting hypoglycemia** (35–50 mg/dL after 6 hours) - **Hepatomegaly** (from accumulation of glucose-6-phosphate → glycogen and fat) - **Marked lactic acidosis** (lactate 8 mmol/L) - **Hyperuricemia** (9.2 mg/dL) — from increased purine degradation and reduced renal urate clearance - **Seizures triggered by fasting** — from cerebral glucose deprivation ## The Primary Metabolic Consequence **Key Point:** Glucose-6-phosphatase catalyzes the **final, irreversible step** of gluconeogenesis: $$\text{Glucose-6-phosphate} \xrightarrow{\text{G6Pase}} \text{Free glucose}$$ Without this enzyme, glucose-6-phosphate **accumulates and is shunted into alternative pathways**: 1. **Glycolysis pathway:** G6P → F6P → F-1,6-BP → pyruvate → **lactate** (explaining lactic acidosis) 2. **Glycogen synthesis:** G6P → G1P → UDP-glucose → **glycogen** (explaining hepatomegaly) 3. **Pentose phosphate pathway:** G6P → NADPH → **fatty acid synthesis** (explaining hepatic steatosis) 4. **Purine synthesis:** Increased PRPP from pentose phosphate pathway → **hyperuricemia** **High-Yield:** The **accumulation of G6P and its shunting into glycolysis** is the PRIMARY metabolic derangement. This explains: - **Lactate overproduction** (pyruvate → lactate) - **Lactic acidosis** (lactate accumulation) - **Hyperuricemia** (PRPP-driven purine synthesis) - **Hepatomegaly** (glycogen + fat accumulation) ## Pathophysiology Flowchart ```mermaid flowchart TD A[G6Pase Deficiency]:::urgent --> B[Glucose-6-Phosphate Accumulates]:::outcome B --> C1[Glycolysis Pathway]:::action B --> C2[Glycogen Synthesis]:::action B --> C3[Pentose Phosphate Pathway]:::action C1 --> D1[Pyruvate → Lactate]:::outcome C2 --> D2[Hepatomegaly]:::outcome C3 --> D3[PRPP → Purines → Hyperuricemia]:::outcome D1 --> E[Lactic Acidosis]:::urgent B --> F[No Free Glucose Released]:::urgent F --> G[Severe Fasting Hypoglycemia]:::urgent ``` **Mnemonic:** **"G6P Trapped = Lactate Trapped = Glucose Trapped"** - G6P cannot exit the liver → accumulates - G6P shunted to glycolysis → lactate accumulates - No free glucose released → hypoglycemia ## Why This Is Different from Other Gluconeogenic Enzyme Defects | Enzyme Defect | Primary Block | Accumulates | Hypoglycemia Severity | Lactate | Hepatomegaly | |---|---|---|---|---|---| | **G6Pase** | **Final step (G6P → glucose)** | **G6P** | **Severe** | **Marked** | **Marked** | | Pyruvate carboxylase | Early (Pyr → OAA) | Pyruvate | Mild–moderate | Mild | Mild | | PEPCK | Mid (OAA → PEP) | OAA | Mild–moderate | Mild | Mild | | F-1,6-BPase | Mid (F-1,6-BP → F-6-P) | F-1,6-BP | Mild–moderate | Mild | Mild | **Clinical Pearl:** G6Pase deficiency is the **only gluconeogenic enzyme defect** that causes: 1. **Severe, life-threatening fasting hypoglycemia** 2. **Marked hepatomegaly** (from G6P shunting into glycogen + fat) 3. **Lactic acidosis** (from glycolytic overflow) 4. **Hyperuricemia** (from PRPP-driven purine synthesis) [cite:Lehninger Principles of Biochemistry 8e Ch 20; Harrison 21e Ch 410]